This invention pertains to a hydraulic control system for off-highway self-propelled work machines such as construction and industrial vehicles, notably including dump trucks and motor scrapers.
The hydraulic control system for the work machines of the type under consideration comprises means for steering the vehicle and means for actuating the implement assembly (i.e., the vessel or bowl in the case of a dump truck or motor scraper). The steering system must be reliable in construction and operation for driving safety and must permit the operator to steer the vehicle in a manner totally unaffected by variations in the speed of the vehicle engine.
The implement actuation system, on the other hand, should provide for the most efficient operation of the implement assembly with as little input horsepower as possible. In work machines such as dump trucks and motor scrapers, in particular, the lowering of the implement assembly requires only less than half as much input horsepower as that necessary for raising it. Conventionally, however, the control system has allowed consumption of the same input power for either raising or lowering the implement assembly, thus wasting a major proportion of the input in the latter instance.
A further objection to the prior art concerns systems incorporating variable displacement pumps for minimal waste of power developed thereby. In dump trucks and similar vehicles wherein the engine speed is subject to change over a wide range, the gain of the control system usually becomes too high during engine rotation at rated speed, possibly giving rise to hunting. This could of course be avoided by setting the gain at a proper value at rated engine speed, but then the setting would be too small at lower engine speed. Combined with the poor response of the variable displacement pumps compared with valves and the like, such too small gain would significantly deteriorate the response of the steering system at low engine speed.